1. Field
This disclosure relates generally to channel sounding and, more specifically, to channel sounding techniques for a wireless communication system.
2. Related Art
In general, orthogonal frequency division multiplexing (OFDM) systems support high data rate wireless transmission using orthogonal channels, which offer immunity against fading and inter-symbol interference (ISI) without requiring implementation of elaborate equalization techniques. Typically, OFDM systems split data into N streams, which are independently modulated on parallel spaced subcarrier frequencies or tones. The frequency separation between subcarriers is 1/T, where T is the OFDM symbol time duration. Each symbol may include a guard interval (or cyclic prefix) to maintain the orthogonality of the symbols. In general, OFDM systems have utilized an inverse discrete Fourier transform (IDFT) to generate a sampled (or discrete) composite time-domain signal.
Wireless networks have generally used an estimated received signal strength and an estimated carrier to interference and noise ratio (CINR) of a received signal to determine operational characteristics of the networks. As one example, IEEE 802.16e compliant mobile stations (MSs) are required to estimate a received signal strength indicator (RSSI) and a CINR of a received signal. The RSSI associated with a serving BS may be used by an MS for cell re-selection and the CINR, which is reported to the serving BS, may be used by the serving BS to adapt a downlink transmission rate to link conditions.
Accurate reported CINRs are desirable, as inaccurate reported CINRs may impact performance of a wireless network. For example, reporting a CINR that is above an actual CINR may decrease network throughput due to frame re-transmission, while reporting a CINR that is below the actual CINR may cause the serving BS to schedule data rates below a supportable data rate. According to IEEE 802.16e, RSSI and CINR estimates at an MS are derived based on a preamble signal, which is an orthogonal frequency division multiple access (OFDMA) symbol that is transmitted at the beginning of each OFDMA frame.
Similarly, wireless networks that employ third-generation partnership project-long term evolution (3GPP-LTE) compliant architectures are required to employ uplink reference signals (RSs) for uplink CINR estimation, which is used by the network to schedule uplink transmission for user equipment (subscriber stations (SSs)). Respective sequences of the RSs are used to uniquely identify an SS and, when transmitted from the SS to a serving base station (BS), may be used by the serving BS in channel characterization. A known channel sounding (channel characterization) approach has proposed limiting a channel sounding bandwidth of cell-edge SSs, i.e., SSs operating at or near an edge of a cell, to reduce interference with neighboring cells and to improve uplink CINR estimation. In this approach, cell-edge SSs sound a portion of a system bandwidth in one sounding symbol and employ frequency hopping to cover the entire system bandwidth using multiple sounding symbols. Following this approach, non-cell-edge SSs are allowed to sound the entire system bandwidth with a single sounding symbol. Unfortunately, the above-described approach generally increases system bandwidth requirements (due to increased scheduling overhead), results in increased inter-cell interference (due to higher power spectral density (PSD) associated with narrower bandwidths), and does not generally improve channel estimation accuracy.
What is needed are techniques for improving channel sounding in a wireless communication system.